By Michael Finkel/Mustang, a former kingdom in north-central Nepal, is home to one of the world’s great archaeological mysteries. In this dusty, wind-savaged place, hidden within the Himalaya and deeply cleaved by the Kali Gandaki River—in spots, the gorge dwarfs Arizona’s Grand Canyon—there are an extraordinary number of human-built caves.

Some sit by themselves, a single open mouth on a vast corrugated face of weathered rock. Others are in groups, a grand chorus of holes, occasionally stacked eight or nine stories high, an entire vertical neighborhood. Some were dug into cliffsides, others tunneled from above. Many are thousands of years old. The total number of caves in Mustang, conservatively estimated, is 10,000.

No one knows who dug them. Or why. Or even how people climbed into them. (Ropes? Scaffolding? Carved steps? Nearly all evidence has been erased.) Seven hundred years ago, Mustang was a bustling place: a center of Buddhist scholarship and art, and possibly the easiest connection between the salt deposits of Tibet and the cities of the Indian subcontinent. Salt was then one of the world’s most valuable commodities. In Mustang’s heyday, says Charles Ramble, an anthropologist at the Sorbonne in Paris, caravans would move across the region’s rugged trails, carting loads of salt.

Later, in the 17th century, nearby kingdoms began dominating Mustang, says Ramble. An economic decline set in. Cheaper salt became available from India. The great statues and brilliantly painted mandalas in Mustang’s temples started crumbling. And soon the region was all but forgotten, lost beyond the great mountains.

Then, in the mid-1990s, archaeologists from the University of Cologne and Nepal began peeking into some of the more accessible caves. They found several dozen bodies, all at least 2,000 years old, aligned on wooden beds and decorated with copper jewelry and glass beads, products not locally manufactured, reflecting Mustang’s status as a trade thoroughfare.

Pete Athans first glimpsed the caves of Mustang while trekking in 1981. Many of the caves appear impossible to reach—you’d have to be a bird, it seems, to gain entry—and Athans, an exceptionally accomplished alpinist who has stood atop Everest seven times, was stirred by the challenge they presented. It wasn’t until 2007, however, that he secured the necessary permits. Mustang immediately became, he says, “the greatest expedition of my life.” This trip in the spring of 2011 was his eighth to the area.

During previous visits Athans and his team had made some sensational finds. In one cave they discovered a 26-foot-long mural with 42 exquisitely rendered portraits of great yogis in Buddhist history. In another was a trove of 8,000 calligraphed manuscripts—a collection, most of it 600 years old, that included everything from philosophical musings to a treatise on mediating disputes.

What Athans and the scientists wanted most was a cave with items from before the era of written records to shed light on the deepest mysteries: Who first lived in the caves? Where did these people come from? What did they believe?

Most of the caves Athans had peeked into were empty, though they showed signs of domestic habitation: hearths, grain-storage bins, sleeping spaces. “You can spend your life looking in all the wrong caves,” says Aldenderfer, whose long career as an archaeologist has included no shortage of frustrating quests.

The ideal cave, he felt, would be one used as a cemetery rather than a home, with pre-Buddhist-era ceramic remains scattered below, on a cliff too high for looters to reach, in a part of Mustang where locals are comfortable with foreigners disturbing their ancestors’ bones. All this, and one additional factor. “Sometimes,” Aldenderfer admits, “you just need to get lucky.”

(This story is a part of Michael Finkel’s extensive report on Mustang Caves published in the National Geographic, October Issue)